Assign delays for 1ms or 2ms in C ?

Question

Hello there,

Im using a code to configure a simple robot. Im using a WinAVR and the code used there is similar to C, but without stdio.h libraries and such, so codes for simple stuff should be entered manually ( e.g. converting dec to hex is a multiple-step procedure involving ascii character manipulation )

Example of code used is ( just to show you what im talking about :) )

.
.
.
    DDRA=0x00;
    A=adc(0); // right hand sensor
    u=A>>4;
    l=A&0x0F;
    TransmitByte(h[u]);
    TransmitByte(h[l]);
    TransmitByte(' ');
.
.
.

For some circumstances, I must use WinAVR and cannot external libraries (such as stdio.h)

ANYWAY, I want to apply a signal with pulse width of 1ms or 2ms via a servo motor.

I know what port to set and such, all i need to do is apply a delay to keep that port set before clearing it.

Now I know to set delays, we should create empty for loops such as :

int value= **??**
for(i=0;i<value;i++);

What value am I supposed to put in "value" for a 1ms loop ?

I would really appreciate if someone can help me with this, and if im posting it in the wrong forum, PLEASE DO TELL ME !!

Thank you very much !

Answer 1

Chances are you'll have to calculate a reasonable value, then look at the signal that's generated (e.g., with an oscilloscope) and adjust your value until you hit the right time range. Given that you apparently have a 2:1 margin, you might hit it reasonably close the first time, but I wouldn't be much on it.

For your first approximation, generate an empty loop and count the instruction cycles for one loop, and multiply that by the time for one clock cycle. That should give at least a reasonable approximation of time taken by a single execution of the loop, so dividing the time you need by that should get you into the ballpark for the right number of iterations.

Edit: I should also note, however, that (at least most) AVRs have on-board timers, so you might be able to use them instead. This can 1) let you do other processing and/or 2) reduce power consumption for the duration.

If you do use delay loops, you might want to use AVR-libc's delay loop utilities to handle the details.

Answer 2

Is this going to go to a real robot? All you have is a CPU, no other integrated circuits that can give a measure of time?

If both answers are 'yes', well... if you know the exact timing for the operations, you can use the loop to create precise delays. Output your code to assembly code, and see the exact sequence of instructions used. Then, check the manual of the processor, it'll have that information.

Answer 3

If my program is simple enough not to need explicit timer programming but should be portable, one of my choices for a defined delay would be AVR Libc's delay function.

#include <delay.h>
_delay_ms (2) // sleeps 2ms

Answer 4

Most ATMega AVR chips, which are commonly used to make simple robots, have a feature known as Pulse-Width Modulation (PWM) that can be used to control servos. This blog post might serve as a quick introduction to controlling servos using PWM. If you were to look at the Arduino platform's servo control library, you would find that it also uses PWM.

This might be a better choice than relying on running a loop a constant number of times as changes to compiler optimization flags and the chip's clock speed could potentially break such a simple delay function.

Answer 5

If you need a more precise time value you should employ an interrupt service routine based on an internal timer. Remember a For loop is a blocking instruction, so while it is iterating the rest of your program is blocked. You could set up a timer based ISR with a global variable that counts up by 1 every time the ISR runs. You could then use that variable in an "if statement" to set the width time. Also that core probably supports PWM for use with the RC type servos. So that may be a better route.

Answer 6

This is a really neat little tasker that I use sometimes. It's for an AVR.

************************Header File***********************************

// Scheduler data structure for storing task data
typedef struct
{
   // Pointer to task
   void (* pTask)(void);
   // Initial delay in ticks
   unsigned int Delay;
   // Periodic interval in ticks
   unsigned int Period;
   // Runme flag (indicating when the task is due to run)
   unsigned char RunMe;
} sTask;

// Function prototypes
//-------------------------------------------------------------------

void SCH_Init_T1(void);
void SCH_Start(void);
// Core scheduler functions
void SCH_Dispatch_Tasks(void);
unsigned char SCH_Add_Task(void (*)(void), const unsigned int, const unsigned int);
unsigned char SCH_Delete_Task(const unsigned char);

// Maximum number of tasks
// MUST BE ADJUSTED FOR EACH NEW PROJECT
#define SCH_MAX_TASKS (1)

************************Header File***********************************

************************C File***********************************

#include "SCH_AVR.h"
#include <avr/io.h>
#include <avr/interrupt.h>


// The array of tasks
sTask SCH_tasks_G[SCH_MAX_TASKS];


/*------------------------------------------------------------------*-

  SCH_Dispatch_Tasks()

  This is the 'dispatcher' function.  When a task (function)
  is due to run, SCH_Dispatch_Tasks() will run it.
  This function must be called (repeatedly) from the main loop.

-*------------------------------------------------------------------*/

void SCH_Dispatch_Tasks(void)
{
   unsigned char Index;

   // Dispatches (runs) the next task (if one is ready)
   for(Index = 0; Index < SCH_MAX_TASKS; Index++)
   {
      if((SCH_tasks_G[Index].RunMe > 0) && (SCH_tasks_G[Index].pTask != 0))
      {
         (*SCH_tasks_G[Index].pTask)();  // Run the task
         SCH_tasks_G[Index].RunMe -= 1;   // Reset / reduce RunMe flag

         // Periodic tasks will automatically run again
         // - if this is a 'one shot' task, remove it from the array
         if(SCH_tasks_G[Index].Period == 0)
         {
            SCH_Delete_Task(Index);
         }
      }
   }
}

/*------------------------------------------------------------------*-

  SCH_Add_Task()

  Causes a task (function) to be executed at regular intervals 
  or after a user-defined delay

  pFunction - The name of the function which is to be scheduled.
              NOTE: All scheduled functions must be 'void, void' -
              that is, they must take no parameters, and have 
              a void return type. 

  DELAY     - The interval (TICKS) before the task is first executed

  PERIOD    - If 'PERIOD' is 0, the function is only called once,
              at the time determined by 'DELAY'.  If PERIOD is non-zero,
              then the function is called repeatedly at an interval
              determined by the value of PERIOD (see below for examples
              which should help clarify this).


  RETURN VALUE:  

  Returns the position in the task array at which the task has been 
  added.  If the return value is SCH_MAX_TASKS then the task could 
  not be added to the array (there was insufficient space).  If the
  return value is < SCH_MAX_TASKS, then the task was added 
  successfully.  

  Note: this return value may be required, if a task is
  to be subsequently deleted - see SCH_Delete_Task().

  EXAMPLES:

  Task_ID = SCH_Add_Task(Do_X,1000,0);
  Causes the function Do_X() to be executed once after 1000 sch ticks.            

  Task_ID = SCH_Add_Task(Do_X,0,1000);
  Causes the function Do_X() to be executed regularly, every 1000 sch ticks.            

  Task_ID = SCH_Add_Task(Do_X,300,1000);
  Causes the function Do_X() to be executed regularly, every 1000 ticks.
  Task will be first executed at T = 300 ticks, then 1300, 2300, etc.            

-*------------------------------------------------------------------*/

unsigned char SCH_Add_Task(void (*pFunction)(), const unsigned int DELAY, const unsigned int PERIOD)
{
   unsigned char Index = 0;

   // First find a gap in the array (if there is one)
   while((SCH_tasks_G[Index].pTask != 0) && (Index < SCH_MAX_TASKS))
   {
      Index++;
   }

   // Have we reached the end of the list?   
   if(Index == SCH_MAX_TASKS)
   {
      // Task list is full, return an error code
      return SCH_MAX_TASKS;  
   }

   // If we're here, there is a space in the task array
   SCH_tasks_G[Index].pTask = pFunction;
   SCH_tasks_G[Index].Delay =DELAY;
   SCH_tasks_G[Index].Period = PERIOD;
   SCH_tasks_G[Index].RunMe = 0;

   // return position of task (to allow later deletion)
   return Index;
}

/*------------------------------------------------------------------*-

  SCH_Delete_Task()

  Removes a task from the scheduler.  Note that this does
  *not* delete the associated function from memory: 
  it simply means that it is no longer called by the scheduler. 

  TASK_INDEX - The task index.  Provided by SCH_Add_Task(). 

  RETURN VALUE:  RETURN_ERROR or RETURN_NORMAL

-*------------------------------------------------------------------*/

unsigned char SCH_Delete_Task(const unsigned char TASK_INDEX)
{
   // Return_code can be used for error reporting, NOT USED HERE THOUGH!
   unsigned char Return_code = 0;

   SCH_tasks_G[TASK_INDEX].pTask = 0;
   SCH_tasks_G[TASK_INDEX].Delay = 0;
   SCH_tasks_G[TASK_INDEX].Period = 0;
   SCH_tasks_G[TASK_INDEX].RunMe = 0;

   return Return_code;
}

/*------------------------------------------------------------------*-

  SCH_Init_T1()

  Scheduler initialisation function.  Prepares scheduler
  data structures and sets up timer interrupts at required rate.
  You must call this function before using the scheduler.  

-*------------------------------------------------------------------*/

void SCH_Init_T1(void)
{
   unsigned char i;

   for(i = 0; i < SCH_MAX_TASKS; i++)
   {
      SCH_Delete_Task(i);
   }

   // Set up Timer 1
   // Values for 1ms and 10ms ticks are provided for various crystals

   OCR1A = 15000;   // 10ms tick, Crystal 12 MHz
   //OCR1A = 20000;   // 10ms tick, Crystal 16 MHz
   //OCR1A = 12500;   // 10ms tick, Crystal 10 MHz
   //OCR1A = 10000;   // 10ms tick, Crystal 8  MHz

   //OCR1A = 2000;    // 1ms tick, Crystal 16 MHz
   //OCR1A = 1500;    // 1ms tick, Crystal 12 MHz
   //OCR1A = 1250;    // 1ms tick, Crystal 10 MHz
   //OCR1A = 1000;    // 1ms tick, Crystal 8  MHz

   TCCR1B = (1 << CS11) | (1 << WGM12);  // Timer clock = system clock/8
   TIMSK |= 1 << OCIE1A;   //Timer 1 Output Compare A Match Interrupt Enable
}

/*------------------------------------------------------------------*-

  SCH_Start()

  Starts the scheduler, by enabling interrupts.

  NOTE: Usually called after all regular tasks are added,
  to keep the tasks synchronised.

  NOTE: ONLY THE SCHEDULER INTERRUPT SHOULD BE ENABLED!!! 

-*------------------------------------------------------------------*/

void SCH_Start(void)
{
      sei();
}

/*------------------------------------------------------------------*-

  SCH_Update

  This is the scheduler ISR.  It is called at a rate 
  determined by the timer settings in SCH_Init_T1().

-*------------------------------------------------------------------*/

ISR(TIMER1_COMPA_vect)
{
   unsigned char Index;
   for(Index = 0; Index < SCH_MAX_TASKS; Index++)
   {
      // Check if there is a task at this location
      if(SCH_tasks_G[Index].pTask)
      {
         if(SCH_tasks_G[Index].Delay == 0)
         {
            // The task is due to run, Inc. the 'RunMe' flag
            SCH_tasks_G[Index].RunMe += 1;

            if(SCH_tasks_G[Index].Period)
            {
               // Schedule periodic tasks to run again
               SCH_tasks_G[Index].Delay = SCH_tasks_G[Index].Period;
               SCH_tasks_G[Index].Delay -= 1;
            }
         }
         else
         {
            // Not yet ready to run: just decrement the delay
            SCH_tasks_G[Index].Delay -= 1;
         }
      }
   }
}

// ------------------------------------------------------------------


************************C File***********************************